Automatic selection of receiving channels



Feb. 13, 1930. w. c. B'OJHN .1',747,z1s

AUTOMATIC SELECTION OF RECEIVING CHANNELS Filed Dec. 11, 1928 2 Sheets-Sheet 1 A 070. Gain Camra/ Can/rel L 66 2 INVENTOR y W. fifioiaa/ BY ?%4 L ATTORNEY Feb. 18, 1930. w, c, BOHN 1,747,218

AUTOMATIC SELECTION OF RECEIVING CHANNELS Filed Dec. 11. 1928 2 sheets-shaft 1i 2 2? Z Razz/We. /?az7o/?ec. I, ,9 .0 1.; imp-De 44:1 am 07)? 1 F/Yt R 3 "3 E m m \3 Q o 4 6. Input x o I .I. C. Z put w I .5 I g 12 i a i y o veratz'zg Current Z INVENTOR WCZBO/Ml/ ATTORNEY Patented Feb. 18, 1930 UNITED STATES PATENT 'oFriciE' WILLIAM C. BOI-IN, OF WYOMING, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK AUTOMATIC SELECTION OF RECEIVING CHANNELS Application filed December 11, 1928. Serial No. 325,197.

This invention relates to methods and means for limiting the extent which any received signal may vary in intensity.

One source of signal variation is that due to changes in the strength of the signal as received from the signal medium. In the case of radio reception such variations are gen erally known as fading. Fading is particularly severe when receiving short wave lengths, and the method generally employed for maintaining a more or less constant demodulated signal has been to provide an automatic gain control or regulator. This method of regulation has certain limitations, however, and becomes inadequate, for example, when the signal drops below the limiting noise level. The present invention has as one of its objects the provision of a system which will be free from these limitations and may be used either independently or supplementary to automatic gain regulation for maintaining a constant output level at all times.

The invention is based on the fact that if a number of receiving stations properly located fore, reduced with increase in the number of receiving stations.

it has been heretofore proposed to use more than one receiver to pick up the radio signal and to choose for use the receiver giving the best results. plished by cutting overmanually from a receiver giving a signal of low volume to an other which at the moment was giving a signal of larger volume, and again switching back to the first receiver or some other receiver as the signal again became low in volume. Obviously, such a method of operation was limited to cases where the changes in the signal occurred slowly enough to permit In the past this was accomthe operator to select the best receiver or receiver output.

Where the signal intensity changes quite rapidly, as is often the case with short wave reception, a system capable at all times of rapidly cutting in the best of the available received signals is desirable. It is therefore proposed to use a number of receiving stations, properly located, and to continuously and automatically select the receiver or output of the receiver giving the most useful signal, minimizing in this way the effect of rapid changes in the amplitude of the received current and reducing the possibility of the signal entirely fading out.

A system to accomplish these results should be capable of automatically, and more or less continuously, selecting for reception purposes the receiver output which is the best of those available, or at least one which is above a predetermined minimum. Where two or more receiver outputs are of equal strength, only one of them should be connected through for reception at any one time. Finally, if while one receiver output is connected through for one reception, another receiver, or receivers, furnish more desirable signals, the system should operate without undue delay to automatically select the signal from one of the more desirable receivers, cutting off at the same time the receiver from which the signal had previously been obtained.

Although various methods for accomplishing the foregoing results may be devised, the systems herein disclosed are so organized that they will depend for their operation upon the amplitude of the demodulated signal currents and will, therefore, have the advantage of being independent of the kind of transmission employed at the transmitter, so that the trans mission may consist either of a single side band, carrier and one side band, or of a carrier and two side bands. At the same time the system may, if it is preferred, be operated by changes in the amplitude of the carrier itself.

The invention will now be more fully understood from the following description, when read in connection with the accompanying drawings, in which Figures 1, 2 and 3 are schematic circuit diagrams embodying three different forms of the invention, while Figs. 4, 5 and 6 are curves illustrating the operation of certain features of the apparatus.

Fig. 1 shows a system for automatically and recurrently selecting the largest or substantially the largest output of one of a series of radio receivers R R R for connection to a main receiving line L. Although only three receivers are shown, the system may be applied to any number of receivers, with slight modification in the relay circuits. In general,

the largest output to be selected may be either I voice or carrier frequency, the design of the delay circuits and of the amplifiendetector system employed depending upon which type of signal is to be selected.

In Fig. 1 the output is shown as adapted for voice frequency operation. It will be observed that the output of each receiver, such as R is connected to a delay device, such as the delay network DN These delay networks are of known construction and have the efiect of delaying the speech currents in a manner similar to that of a. long wire line circuit. In the presentinstance, the delay circuits are used to delay the speech currents long enough to permit all of the associated control relays to function before the speech currents arrive at the short-circuiting contact, such as a which controls the connection of each receiver to the line L. In order to control the connection of the receiver R to the line L, a filter F is provided for selecting some of the voice energy'from the output of the receiver R (In case the automatic control is to be exercised from the fading of the carrier, the filter F would be designed to select the carrier frequency.) On the output side of the filter F is an amplifier-detector unit AD of well-known construction, for amplifying and rectifying the selected energy to produce a direct current whose instantaneous amplitude is determined by the amplitude of the signal received from the receiver R This rectified current is used to control the relay switching arrangements to be described hereinafter.

These switching arrangements are primarily controlled by a master relay MR in the output of the amplifier-detector AD WVhen a signal current is received from the output of the receiver R of sufiicient volume to operate the master relay MR the armature of said relay is attracted to release the slow release relay TR which in turn completes the circuit of the hangover HOR The latter relay, by opening the short-circuiting contact a removes the short-circuit from the output side of the delay network DN thereby establishing a connection through the receiver R to the line L and permitting the speech currents which have been delayed by the network DN to pass to the receiving line. The

hangover relay HOR is a slow release relay and thereby introduces a sufiicient hang-over operation to enable any currents which are just entering the delay network at the end of the signal, to pass through the network to the receiving line under the control of master relay MR As will be pointed out later in connection with the circuit arrangements whereby one of the receivers is selected to the exclusion of the others, conditions may arise which will permit a momentary operation of the master relay under conditions such that some other receiver is ultimately to be selected. WVhile in general such momentary operation may not produce any harmful effect, since the hangover relay will have released before the pulse of signaling current producing the momentary operation of the master relay MR will have passed through the delay network DN the relay circuit arrangement of Fig. 1 is so arranged as to prevent any opening of the short-circuit contact a in response to a momentary operation of the master relay MR This is due to the fact that the intermediate relay TR is slow to release and when the contact of the master relay MR is only attracted momentarily, the armature of the intermediate relay TR is not released to close the circuit of the hangover relay HOR to in turn open the contact a In order to properly control the various selecting relays and circuits whereby a particular receiver is selected in such a manner that the receiver having the largest output will be selected in preference to a receiver having a lesser output but sufficient to operate its master relay, the master relay, such as MR associated with receiver R should operate more quickly in response to a large rectified current than in response to a smaller rectified current. Since the ordinary relay responds more quickly to larger currents than small currents, this requirement does not call for any special design of the master relay MR Fig. 6 shows a typical curve illustrating the relation between operating current and operating time for a master relay. 0w represents the minimum current necessary to operate the master relay. Until the received signal from a particular receiver is of sufiicient strength to produce a rectified current of this value, the master relay'will not operate and hence the receiver cannot be connected tothe receiving line L, WVith the rectified current of the value 0w, however, an operating time mx. will be required to operate the master relay. If a greater operating current, such as 0 z is received, the relay will only require the length of time yg to operate. If the operating current should have the value 0z, the time required to operate the relay would be that represented by 2a.

In order to have the best operation of the system so that the discrimination between different receivers, with respect to the time of the operation of the master relays, may be more exact, it might be desirable to have an amplifier-detector arrangement of such character that a linear relation would exist between the input signal current and the operating time as illustrated by the curve of Fig. l. If it is considered desirable to maintain this strict proportionality between the operating time and the input signal current, the detector, in the amplifier-detector such as AD, may be similar to the well-known design employed in connection with volume indicators, such as are used in broadcasting stations. The characteristic of such a detector will be similar to that indicated by the curve of Fig. 5. In actual practice, however, the relation between the relay operating time and the signal input current shown in Fig. 4 need not necessarily be linear so long as the operat ing time decreases with increase of the input current. The ordinary amplifier-detector such as is used in connection with echo suppressors will be satisfactory for the purpose of the present invention, and in the following description it is assumed that the amplifierdetector units, such illustrated at AD are the echo suppressor type.

The apparatus so far described is similar for each receiver and hence the corresponding apparatus associated with the receivers R and E is not described in detail.

In order that the proper receiver may be selected at any given time, it is necessary that a master relay, such as MR operate certain dependent relays for controlling the selection. For example, if the transmitted signal is received simultaneously by the three receivers illustrated and each has approximately the same sensitivity, and each is connected to a similar antenna, then if the antennae are sufficiently far apart, the three received signals will in general not fade simultaneously. At any instant, therefore, the signal may be maximum on receiver R or on receiver R or on receiver R or, again, the amplitudes may be the same on the receivers R and R or R and E or R and R but greater than the amplitude of the third receiver; or the signals received on the three receivers may be of the same amplitude. In order to efiect a proper selection under any of the above conditions, relays CR CR and CR are associated with the several receivers, each of these relays being operated whenever the corresponding master relay attracts its armature to its front contact. Each of the relays, such as CR is designed to be quick in its response so that its selecting functions may be performed without delay.

Each of the master relays may be disabled by means of short-circuit connections, such as C C and C The relay CR has two 7 contacts 6 and 0 the former of which closes the short-circuited C for the master relay .and 0 controls the short-circuiting connections for the master relays MR and MR respectively. Likewise, by means of its contact 6 the relay CR controls the circuit of the auxiliary relay AR to disable the shortcircuitC of its own master relay MR by means of the contact 2. The relay'CR by means of its contacts and 0 controls the short-circuit C and C respectively, for disabling the master relays MR and MR Said relay CR however, is provided with no auxiliary relay and hence cannot open the short-circuit C associated with its own master relay. Consequently, as will be described later, if the receiver R and any other receiver are receiving signals of the same strength simultaneously, the other receiver will be given preference.

The selecting arrangement above described is so designed that, when it selects, it selects the maximum signal for connection to the main receiving line L at all times to the exclusion of signals from any of the other receivers. If two or more receivers yield approximately the same maximum signal, only one isconnected through for reception. Advantage is taken of the fact that the time required for a master relay to operate depends upon the amplitude of the operating current. The operating time of the master relays employed will therefore depend upon the rectified current supplied to them by the amplifiendetectors which, in turn, depends upon the output speech levels of the receivers across which the amplifier-detectors are bridged. l Vhile, as already stated, this relation between the out-put speech level and the operating time may be made prcatically linear, if desired, a non-linear speech level versus relay operating time characteristic is in practice sufiicient to enable the selective system to readily select between signals which are above a predetermined minimum intensity. It will not, however, select the maximum signal at all times since the differential between two or more very strong signals from different receivers will not be as fine as between weaker signals furnishing detector currents which are just above the minimum operating current necessary to cause operation of the master relay. A receiving system employing such non-linear selecting arrangement will, however, effect a considerable improvement over the conventional method of receiving signals since signals of usable intensity will be continually selected, and the possibility of the signal fading out will be greatly reduced.

The nature of the selecting arrangement is also such that whenever the signal from a particular selected receiver falls below the value necessary to cause operation of its master relayin other words, whenever the signal from that receiver fades below a given value, the selection effected under the control of its master relay will no longer be effective, and a new selection will be started. Also, a new selection will occur whenever a momentary interruption occurs in the signal, such as the pause between sentences and, in fact, the system may be so designed as to re-seleet whenever a pause takes place such as would occur between successive words. Therefore, while a receiver once selected will remain selected so long as its output is above the minimum requirement, even though another receiver may a moment later have a greater output, re-selection due to pauses will occur with such frequency that in general the signal from a particular selected receiver will not have time to fade materially.

In any event, the volume of the signals connected to the main receiving line can be kept substantially constant by means of an automatic gain control such as is indicated schematically at GO in Fig. 1. This gain control arrangement may be of any type well known in the art arranged to respond to the received voice range as distinguished from the carrier. Such a gain control arrangement is illustrated, for example, in the patent to Espenschied and Bown, No. 1,447,773, of March 6, 1923. Automatic gain controls have heretofore been used with some success for accommodating the gain or sensitivity of a receiver to the signals received. However, when the signals fade down into the noise levels and are brought up by the introduction of more gain by the automatic gain control, the noise is brought up by the same amount so that the signal may still be unintelligible. This disadvantage may be overcome, however, if maximum signals are selected from a plurality of receivers, as above described, and the volume is controlled in the main receiving line, as indicated in Fig. 1. Since the volume of the signals from the different receivers determine which receiver shall be cut through to the receiving line, the range in volume which the automatic gain control in the receiving line must care for will be com paratively small.

OPERATION on FIGURE 1 Further details of the apparatus of Fig. 1 will be clear from the description of its operation, which is as follows:

Useful signal from one receiver only Let us assume that the largest signal output is being received from receiver R, and that the signal outputs from the receivers R and R are not suflicient to cause the operation of the corresponding master relays MR and V circuit from the output of delay circuit DN at a This action takes place just before the signal is passed through the delay network and the receiver R is now out through to the line L. The relay CR at its upper front contact e completes the circuit of the auxiliary relay AR which at f, opens the shortcircuitconnection 0 which might be established across the master relay MR by the relays CR and CR in case they should be operated. At the same time the relay CR- closes its contacts I), and 0 the former clos ing the short-circuit connection C about the master relay MR associated with the receiver R and the latter closing the short-circuit C about the master relay MR associated with the receiver R The output of the receiver R is now connected to the line L exclusive of the other receivers, and should the volume of the output of either of the other receivers increase to such an extent as to be able to operate corresponding master relays, such operation would be prevented and the receiver R would remain exclusively. connected to the line L until such time as the master relay MR- is released, due to the fading of the signal from the receiver R or due to an interruption in the signal between successive words or sentences (or, in the case of telegraph transmission, between successive telegraph impulses), of suflicientduration to enable MR to release the hangover relay H011 and again shortcircuit the connection L between the receiver R, and the line L.

If either receiver R or receiver R should be the only receiver whose output was of sufficient magnitude to operate the corresponding relay, the operation of connecting such receiver to the line L, to the exclusion of other receivers, would be similar to that justdescribed.

Maan'mwm signals from one receiver only here more than one receiver supplies an relays.

other, the receiver having the larger output will be selected and connected with the line L. For example, if the signals from receivers R and R are strong enough to operate the master relays MR and MR but the signals from receiver R are stronger, master relay EH25 will operate first. The operation of this relay sets into operation a train of relays, including hangover relay HOR to remove the short-circuit from the line L at contact a As soon as the master relay MR is operated, it also actuates relay CR to short diate relay TR prevents the release of the hangover relay HOR in response to a momentary actuation of the master relay, as has already been described. Similarly, a momentary actuation of the master relay MR before the short-circuit C is applied by the contact 6 would not result in the operation of the hangover relay HOB to out the receiver R through to the line L.

On the other hand, if the signal from the receiver R is such that the relay MR while operated an instant later than the relay MR is operated so nearly simultaneously therewith that the relay CR has time to pull up and open the contact a, before the contact at a, can effectively establish the circuit of the relay AR the relay AR, will be prevented from operating to open the possible shortcircuit connection for the relay MR at f The radio receiver R may now again control the circuit by reason of the fact that relay CR energizes relay AR to open the possible short-circuit of the relay MR at f Contacts 6 and 0 in the meantime short-circuit the master relays MR and MR so that the relay MR will fall off and release the relay CR While the receiver R is now connected exclusively to the circuit notwithstanding that the amplitude of the signal from the receiver R is slightly greater, this action is only possible where the margin of difierence between the amplitudes of the two signals is relatively small, with a consequent small difierence in time discrimination between the operation of the two master The improper selection above described could not take place where the opera tion of the relay MR was sufiiciently later than that of the relay MR to enable the latter to establish the short-circuit at 6 across the relay MR before it had been actuated sufliciently to energize the relay CR No particular harm would be caused by such improper selection for the time margin of operation of the two relays would depend upon a marginal difference between outputs of the .two receivers of such relatively small magnitude that it would not really be important to select the largest signal. In any event the fact that a signal of slightly smaller volume than the maximum had been selected, would be adequately compensated by the action of the auto-gain control apparatus GO.

Signals from two receieers equal to and stronger than those from the third Suppose the signals received from receivers R and R are of equal strength and greater than the signal from receiver R Under these conditions the master relays MR and MR operate simultaneously. Relay MR completes the circuit of relay CR so that contacts 6 and 0 short-circuit the master relays MR and MR Relay MR also completes the circuit of relay CR but the closing of the contact 22 by the latter, which would normally put a short-circuit on the master relay MR is prevented by the fact that the relay CR is actuated to complete at 6 the circuit of the relay AR and open the possible circuit of the relay AR which might be closed by the relay CR The relay AR opens at f the short-circuit which would otherwise be established by the contact 6 The relay A13 cannot operate while the relay CR is operated. Hence while the contact 6 disables the relay MR associated with the receiver R the contact 0 cannot disable relay MR associated with receiver R due to the operation of relay AR; with the consequent opening of the contact f Therefore, only the signal from the receiver R is connected through to the line L by the opening of the contact a on the part of the hangover re lay HORP In general, whenever the signal received from the receiver R is equal to the maximum signal from any other receiver, as in the case just described, the signal from the receiver R is given preference, due to the action of the relay AR and its contact f which prevents disabling of its relay system while permitting the operation of its own relay system to disable the operation of the relay system associated with the other receiver. Therefore, when maximum and equal signals are received on the receivers R and R or the receivers R and R the signal from the receiver R is preferred in the selection and cut through to the receiving line. The same holds true when the signals from all three receivers are equal.

When the signals received from the receivers R and R are maximum and substantially equal, the master relays lVlR and MR will operate simultaneously. The relays CR and CR will also operate simultaneously, the former to short-circuit MR at the con tact c and the latter to short-circuit MR at the contact 6 The relay CR however, completes the circuit of relay AR at the contact e thereby causing the opening by the conalready described. The relay MR associated with the receiver R is of course held inoperative by the closing of the contact 6 Re-seZec-tz'on It will, of course, be obvious that when one of the receivers has been selected and connected to the line L, it will remain so connected to the exclusion of other receivers, even though its output may thereafter become less than that of some other receiver, until the master relay associated with the particular receiver is released. This will, of course, take place whenever the signal fades to an amplitude lower than necessary to operate the master relay. It will also take place whenever an interruption occurs in the signal, such as might result from the pause between two sentences or the pause between successive words of a sentence. In the case of tele raph operation, such release of the master relay might take place between successive signal impulses. As soon as the relay releases, the master relays of the other receivers become operated again so that the most desirable signal can again be selected. If the output of a different receiver is se lected at this time, the a-contact of the hangover relay associated with the receiver previously used will be closed to disable the receiver just before the signals from the newly selected receiver have had time to pass through their delay circuit.

The rate of re-selection of course depends upon the time of operation and release of the various mechanical relays. With relays of the type available, which will operate in from .003 to .005 of a second or even less, the. operation of the chain of two or three secondary relays, which depends upon the operation of the master relay, would not require a total operating time of much more than .01 of a second and perhaps even less. Under these circumstances new signals could be selected as often as one hundred times per second without interrupting the signal to the line L.

The sensitivity to which the voice-operated devices can be adjusted will depend, as in the case of all devices of this kind, upon the prevailing noise or static, for if the device is too sensitive, noise may cause false opera tion. This limitation, however, does not detract in any appreciable degree from the utility of the selecting system above described, especially when it is used for short wave reception Where static is usually of the steady type and well below the signal level. Signals nearer the steady noise level would in most cases be uncommercial even if they could be selected, and one of the chief advantages of multiple reception and selection lines, of course, in the fact that a means of avoiding such signals, as much as possible, is afforded.

Obviously, provision may be made for keeping the receiving line L connected through to the air by way of another receiver at all times when the receiving line is not so connected through one of the three selective receivers shown. The use of such a stand-by receiver would not, in general, be necessary, as the several selective receivers provided will ordinarily be sufficient to insure that some one of them will practically always afford an output of sufiicient amplitude to actuate its master relay and thereby connect it to the line.

OPERATION or FIGURE 2 CR and CR which are controlled by the master relay, are given different hangover periods. These hangover periods need not be of long duration, the essential requirement being that they be different.

The operation of the circuit under conditions when the output of only one receiver is of sufiicient amplitude to operate a master relay, is the same as described in connection with Fig. 1 and need not be discussed further. Also, the operation will be as previously described when more than one receiver supplies an output sufiicient to operate the corresponding master relay, but the output of one receiver substantially exceeds that of the other receiver.

When signals from receivers R and R are maximum and equal We will now consider the operation of the system when the signals received from receivers R and R are maximum and equal,

and hence the master relays l 1 and MR- associated with these receivers operate simultaneously. Relay CR is operated by master relay MR and at its contact 6 places the short-circuit across master relay MR Master relay MR has, however, in the meantime completed the circuit for relay CR which at its contact 6 places a short-circuit across the relay MR As the relay CR has no hangover, or substantially no hangover, it is immediately deenergized following the short-circuit placed upon the master relay MR so that the contact 6, is opened and the short-circuit is removed from the master relay MR permitting the latter to be acted upon by the incoming signals once more. Relay CR having a considerable hangover period, does not open its contact 6 in response to the momentary short-circuit ap plied to the master relay MR and hence the short circuit established across the relay MR remains, with the result that the receiver R is connected through to the line L by removing the short-circuit at (a just before the signal passes through the delay network DN The receiver B on the other hand, is not connected through to the line L, as the momentary actuation of the master relay MR does not cause the hangover relay HOB to operate.

The time required to free the master relay ME so that it can again be acted upon by means oi the signals, is equal to the release time of the relay CR which would be about .003 seconds or less. The hangover time of the iay CR therefore, ought to be slightly r than that in orderthat the master relay MR be kept short-circuited until the signal currents have had time to operate the master relay lvlR again. The delay device BN must, of course, delay the signals until all of the foregoing operations are completed. The total time required for this entire operation can be made of the order of .01 second or less when fast mechanical relays are employed.

in the case of l, the hangover relays H03 etc., should be so controlled by the master relays that they will not be operated by a short pulse through the master relay. The initial pulse which operates both the relays MR and MR will therefore be of too short duration to make the hangover relay HOR operate and hang over to falsely connect signals from receiver 3 through to the receiving line after llfij have passed through the delay device BN By this circuit arrangement it is possible in all cases to prevent the signal from more than one receiver being connected to the receiving line L at any one time. Hence, when the relay MR is operated the secoond time and the contact 6 can no longer interfere with the continued operation of the relay M3,, by speech signals (due to the hangover of relay CR the hangover relay E0132 becomes operated and connects the signal through, While contacts 6 and 0 disable the master relays associated with the receivers it, and li The hangover period of the hangover relays, such as l-lOR HOR etc., must, of course, be of sutlicient duration to permit all of the delayed speech currents to get through to the receiving line after the cessation of speech or other signals.

Re-se'lectlon W 1726 receivers R and R receive equal maximum signals The operation under this condition will be similar to that described for the condition when equal maximum signals are received from receivers R and R The relay CR is given a somewhat larger hangover than the relay CR with the result that relay CR hangs over during the brief period that the relay CR is energized and applies a short circuit to the master relay MR at 0 Relay CR therefore, maintains the short circuit applied to the master relay MR at contact 0 thus giving the signal from the receiver h, the right of way.

When signals from all three receivers are equal and of swfilcient strength to operate the master relays Under these conditions, all three master 1 relays are simultaneously operated. Relays MR and MR however, are only operated momentarily and the corresponding control relays CR and CR therefore, maintain the short-circuit across the master relay MR which is controlled by the contacts 6 and 0 until both of these control relays release. As the control relay CR has a slightly longer hangover than the control relay CR the latter releases first, thereby removing the shortcircuits applied to master relays MR and MR at a, and 0 A moment afterwards the relay CR releases and removes the short-circuits applied to relays MR and MR at contacts l), and 0 No short-circuit is now applied to the relay MR and as the relay CR has a slightly longer hangover than relay CR it does not release, due to the temporary application of the short-circuit to the relay MR but maintains the short-circuit applied at contacts 6 and 0 to short-circuit the master relays MR and hilt- Consequently, signals will be received and applied to the line L only from the recevier R It will, of course, be obvious from the foregoing that if equal maximum signals are received from receivers R and R and a signal of materially less volume is received from receiver R both receivers R and R, will be given preference to R due to the fact that their master and control relays will be energized toshort-circuit the master relay R before the latter will have had an opportunity to energize the control relay CR As between receivers R and R from which signals of the same volume are being received, receiver R is arbitrarily given preference, due t) the longer hangover of its control relay OPERATION or FIGURE 3' As in the case of the systems previously described, the arrangement shown in Fig. 3 may be designed either (1) simply to select one of a number of signals which may be above a certain predetermined minimum intensity or (2) to select the signal having the maximum intensity. WVhether the system will operate the one way or the other depends upon the characteristic of the amplifier-detector units employed. If these units have an operating characteristic such that the time they will require to operate will depend inversely upon the strength of the signal impressed upon them, the circuit is such that the receiver giving the strongest signal will be selected by virtue of the fact that the master relay actuated by the strongest signal will operate faster than the master relays actuated by the weaker signals. The range and nature of this relation of relay operating time versus actuating signal which would be required, depend upon the range over which the signals to be selected may vary in intensity (due to fading, etc.) and upon how closely the apparatus shall be required to differentiate between signals of nearly the same amplitude. I

If, on the other hand, the apparatus is designed simply to select signals above a predetermined minimum intensity, not necessarily the maximum of the received signals, the operating time versus operating signaling intensity characteristic is of less importance so long as the sensitivity of the voiceoperated device is adjusted, so that it will positively operate on signals above the predetermined minimum intensity.

The apparatus of Fig. 3 is in general similar to that of Figs. 1 and 2 except in so far as the circuit arrangements for determining preference as between different receivers are concerned. As these structural and circuit differences will be evident from the description of the operation which follows, no detailed description of the apparatus and circults Wlll now be glven.

Useful signals from one receiver only Let us assume that the receiver R only is furnishing a signal strong enough to operate a master relay and that the master relay MR is operated thereby. Relay MR through relay TR and hangover relay HOR operates the contact al in a manner previously described in connection with the other figures to connect the signals from the receiver R to the receiving line L through the delay network DN The master relay also completes a circuit through the upper winding of relay CR This relay is provided with a lower biasing winding and is constructed in a manner well known in the art so that current through the upper or operating winding tends to attract its armature and thereby close the contacts g and 8 controlled thereby while current through the lower or biasing winding tends to repel the armature and thereby open the contacts. This is indicated by the two arrows applied to the relay.

When current flows through the upper winding of relay CR due to the action of the master relay MR contacts and 8 are closed. Contact 9 connects battery through the upper winding of the disabling relay DB associated with the receiver R through the winding of an auxiliary relay XR through the winding of the disabling relay DB and through the lower or biasing winding of the relay CR associated with the receiver R Relay DB is also provided with a second winding whose circuit may be closed over the contact 9 of the control relay CR This second winding produces the same effect upon the armature of the relay as the upper winding, this fact being indicated by the arrows in the figure pointing in the same direc tion. The current flow through the upper winding of relay DB causes contact to close and short-circuit the receiver R Likewise the current flow through relay DR causes contact 10 to close and short-circuit the receiver R The energization of the auxiliary relay XR and the current through the biasing winding of relay CR are without use or effect at this time. As a result of the foregoing actions, master relays MR- and MR are prevented from subsequent operation on peak values of speech or other signals, and signals from the receiver B only are connected through to the line L for reception.

Returning to relay CR the closing of the contact 8 at this time is of no importance as the function of this contact only comes into play usefully when signals from receivers R and R are strong enough to operate relays MR and MR before the disabling relays DB and DR, have had time to short-circuit the master relays through contacts 1 and 116 respectively. Since we are here considering only the simplest case where signals from receiver R1 alone are strong enough to operate master relay MR master relays MR and MR are not positively'operated, and hence the function of the contact 8 which is the short-circuit to thereby prevent the operation of relay DB need not be present. '4

Obviously, the operation is similar to that above described when any one of the other re- .ceivers alone receives signals strong enough ,to operate its master relay.

Mawlmum signals from one receiver only When signals from receivers R and R are strong enough to operate master relays MR and MR but the signals from receiver R are stronger, master relay MR will operate be fore either master relay MR 6M11 due vto the characteristics of the amplifier-detector units and the master relays as illustrated in Figs. and 6, Relay MR causes the current to flow through the upper winding of relay CR thereby closingc'ontacts s and 9 before either relay CR or CR has had time to operate. Contact 8 "short-circuits relay .and the biasing winding of relay CR so that the subsequent operations ofrelay GR or CR cannot. cause vthe closing ofcontactk (which would short-circuit the master relay M3,) or permit the bias to interfere with the actuation of relay (1R The closing of contact 9 in the meantime operates relay DR through its upper winding andalso operates relays X11 and DB thereby short-circuit ingiat I0 and Z0 themaster relays MR and Also, thehangover relay HOB which hadbeen positively operated by the master relay MR cuts the signals from the receiver B1 (which are the stronger) through for reception.

It ,mightbe here pointed out'that as in the case of Figs; 1 and 2, the contacts a a and a are so controlled that they will not be Operated by a very short iInpulse through the roaster relays MR MR and MR For eX- ample, such a pulse mightbe given relays and MR before thecontacts le and have been closed in response to the actu ation of control relay CR as abovedescribed. Therefore, if the master relays MR and MR are momentarily operated, no harm isdone from the standpoint of the possibility ofthe signal being simultaneously received gfrom two recelvers. No s gnal would be received from IeC6lV6IS R and R in this case s incethe delay networks are arranged to delay the signals long enough for all relays to .com-

.plete their operations before the signals have fha d time to pass through the delay network.

Only the stronger signals from the receiver R therefore, get to the receiving line.

the. weaker signals from the receivers R and 13 having heen excluded. The operation will,

of course, be similar if at the time of the selection, signals from the receiver R are definitely stronger than those from receivers R and R and the same holds true in the case where the signals from the receiver R are definitely stronger than those from the other recelvers.

fie-selection So long as the signals from the selected receivers are stronger than the signals from other receivers, the selectedreceiver will continue to be connected to the receiving line L. When the continuity of the selected signals is interrupted, as, for example, by the normal keying of telegraph signals, or momentary cessation of speech, or by the fading of the received signal, the master relays and dependent relays at once release so that reselection of signals is possible. The interval of interruption required to permit re-selection to take place will depend upon the release time of the master relays. If these relays release rapidly, a short interruption, something like .005 second will permit reselection. At telegraphic signaling speeds of i 100 words per minute, the average interval between dot-dash characters making a single Morse letter is about .012 second or about four times longer than would be required to cause the master relays to release. Re-selection could, therefore, easily take place in the interval between any dot or dash in the transmission. On speech modulation, intervals of the required order continually occur between sentences and words and oftentimes on Weak syllables.

:If, therefore, the selected signal is at any time lower in amplitude than some other signal, the signal of higher amplitude will al- .most immediately be selected. The hangover relays, such as HOR are adjusted to maintain their contacts, such as a closed until the last element of the disconnected signal has been passed through the delay network. Should the selected signal suddenly fade before the selecting system has had an interruption on which to re-select, the master relay would release as soon as the signal dropped to less than the required minimum, permit- When signals from two receivers are equal and stronger than those from the third andR are the strongerand are of equal amplitudes. Under these conditions master relays MR and MR operate simultaneously, causing control relays CR, and CR to operate simultaneously, with the consequent simultaneous closing of contacts 9 8 9 and 9 Contact 8 prevents battery over contact 9 from energizing relay DB and the biased winding of relay CR Contact 9 however, closes the circuit through the upper winding of relay DR and through the windings of relays XR and DB and also through the biasing winding of CR Contacts and 70 are therefore closed to disable receivers R and In the meantime contact a which has been closed by the hangover relay I-IOR in response to relay MR cuts the signals from the receiver R through to the re ceiving line L while the signals from the remaining receivers will continue to be shorted out. During the momentary operation of contact 9 simultaneously with the operation of contact 9 (and before contact 9 caused the contact 70 to cut the operating signal from the amplifier-detector AD contact a would not be operated and would not hang over, due to the provision of the slow release intermediate relay TR between the master relay and the hangover relay, as previously described.

The operation is similar when signals from receivers R and R are equal, the signal from receiver R being chosen in preference to signal from the receiver R in that case, by reason of the fact that the contact 8 is provided in connection with the control relay CR thereby short-circuiting relay DB and preventing its energization inresponse to the energization of the relay CR The relay CR has no corresponding protecting contact for its corresponding disabling relay DB It might also be mentioned that the auxiliary relay XR is not operated over the upper contact 9 of relay CR since the circuit does not pass through relay XR as it does in the case of the corresponding contact associated with the receiver R When signals from two receivers are above the required minimum but not equal Let us now consider the operation for the case in whlch the signals from receivers R and R are of suflicient strength to operate corresponding master relays but the signal from the receiver R is the stronger. In this case relay CR will be operated before the relay CR and hence disables the selective apparatus associated with the receiver R so that only signals from the receiver R can be connected through for reception by the line L. Contact 9 of relay CR puts battery through the winding of the relay DB and through the biasing winding of relay CR Relay DR is energized and by means of contact k short circuits the receiver R and thereby disables the apparatus associated therewith, including the master relay MR The current flowing through the biasing winding of the relay relay CR acts as a strong restoring force to reverse the direction of travel of the armatures g and 8 should they be traveling toward their front contacts at the time contact 9 of relay CR is closed. In other words, if contact 9 closes first, due to stronger signals from the receiver R operation of contact 9 and 8 may, due to the bias, be prevented from operating falsely immediately after.

Other and perhaps more effective means might be'designed to prevent such false operation, but as will be clearly apparent, little harm is done if contacts 8 and 9 should close just after the contact 9 7 Let us consider the case, for example, where the armatures s and 9 are too near their front contacts for a good strong biasing force to succeed in preventing them from contacting, after the contact (or 9 has placed battery on the biasing winding of the relay CR Contact 8 would remove battery from relay DB and the biasing winding of the relay CR while contact 9 would, through relays DB and DB disable re ceivers R and R Under these conditions the signal from receiver R would be selected instead of that from receiver R In such case, however, the signals from the two receivers would be of so nearly the same amplitude that it would make substantially no difference which of the two were'selected.

In the case just assumed, a momentary actuation of relay DB might cause the contact to be closed and thereby momentarily disable the receiver R In order to prevent the contact 70 from disabling the receiver R (and hence, the relay MR momentarily, relay DB may be made just sluggish enough so as not to operate on pulses which are not sufficiently strong to enable the bias to prevent the armatures s and 9 from contacting after 9 (or 9 With the hangover relay, such as HOR adjusted to release the moment the last element of the signal has passed through the associated delay network, the corresponding disabling relay, such as DB need not be depended upon to cut the signal off the line L the moment another signal has been selected.

The operation is similar if the signals from the receivers R and R are maximum and .the signals from receiver R are the stronger.

Here the control relayCR operates before the control relay CR and through its contact 9 connects battery through the winding of disabling relay DB and the biasing winding of the relay CR If the signal from receiver R is sufficiently strong so that the circuit through the biasing winding is established ,with sufficient rapidity as to cause the bias to repel the armatures s and 9 the receiver R will be selected. If the signal from the receiver R is only slightly stronger and almost equal to that from receiver R the signal from the latter may be selected in preference by a method analogous to that ust described in connection with receiver R Let us next consider the process of selection, first, when signals from the receivers R and R are equal, and second, when the signals are nearly equal. WVhen signals from R and R are equal, relays CR and CR are operated simultaneously and hence the contacts {/1 and s and g and 8 are closed simultaneously. Contact 8 prevents the en ergization of the relay DR in the biasing winding of relay CR Contact 9 operates relay DB through its upper winding and closes the contact 76 thereby short-circuiting the receiver R WVith the contact 8 closed, the winding of relay DB and the biasing winding of relay CR are short-circuited and the battery connection over the contact 9 through the upper winding of relay DB passes through the winding of the auxiliary relay XR and thence over the contact s and the back contact of relay XR to ground. This causes the operationo'f the auxiliary relay XR without operating relay DB and the biasing winding of relay CR The operation of the auxiliary relay XR opens by means of its armature the short circuit which previously existed across the winding of the relay DB and the biasing winding of the relay CR due to the closing of the contact .9 Relay XR however, continues to be operated in series with the winding of the relay DB and the biasing winding of the relayCR The energization of relay DB with resultant closing of contact 79 short-circuits the receiver R thereby disabling it and giving preference to the signal from the receiver R Consider now the case when the signals from receivers R1 and R3 are not quite equal, and assuming that the signal from receiver R is but slightly stronger than that from receiver R so that the contacts 9 and 8 cannot beprevented by the biasing action of the lower winding of relay CR from closing just after contact 9 has been closed by the relay CR In this case the contact 9 will operate the relay DB and the auxiliary relay X11 so that the latter removes the short-circuit placed by contact 8 across the winding of the relay DB and biasing-winding of relay CR Consequently, relay DB will be energized and by closing contact will short-circuit the receiver R and give the signal from receiver R the right of way. This case is analogous to that just described for signals from receivers R and R which are not quite equal, and the reasons for justifying the selection of the slightly weaker signal hold true here.

Operation when all signals are equal When the signals from all three receivers are equal, all three master relays will be 0perated simultaneously, and hence, the control relays CR CR and CR will be operated simultaneously. Contact .9 operates DB XR DB and the biasing winding of relay CR Relay DB at contact la short=circuits receiver R Auxiliary relay X11 opens the short-circuit across relay DB and'the biasing wind-ingot relay CR which would otherwise he closed at contact 8 of relay CR vRelay DB being energized, short-circuits the re.- ceiver R at cont-act 70. At receiver R the closing .of the contact 8 of the relay CR shortrcircuits the relay DB and the biasing winding of relay CR and therefore prevents the receiver R from :beingdisabled. As soon as the hangover relay HOB. operates, the signal through the receiver R passes from the delay network DN to the line L,.and receiver R1 is exclusively selected.

When the signals from the three receivers are not exactly equal but have so nearly the same amplitudes that the currents in the biasing windings of relays CR1 and CR3 cannot prevent the contactsot said relaysfrom being closed, the signal from the receiver :R is always chosen. This follows iroln the :fact that the moment contacts 9. and 6. are operated, whether before or after the operation of contacts g and g ,and g and 8 the contact 91 causes the .short-circuiting of the receivers R andR -in themanner previously described, While the contact .8 by protecting {the relay DB and the biasing winding of relay .03 gives the signals from the receivers R1 the right of way. Here, again, it will he not d that even though the signal from r ei er R or R3 might :be slightly greater than tha from receiver R the three signal will have so nearly the :same amplitude, that it will he a matter of no moment whichis chosen.

It will be .obvious that the general principles herein disclosed ma-y embodied in many other organizations widely differen from those illustrated, Without departing from the spirit ogtrthe intention as defined in the following claims.

What is claimed is:

1. In a signaling ystemzsubject it 'rariah transmission conditions, a plurality o r ceivers each receiving the sam signa and so related to each other that the received-signals vary differently at the seneral receive-rs with variations in transmission conditions, a receiving circui and means associated with said circuit to -automatically;selo t receiver whose signal output exceed a :pr a signe minimum,

2. In a signal-ing systeaisnb-jecti v riabl transmission conditions, a :pl re lity iogt receivers each receiving theesi me Briana-land so related to each other-that the receivedsignals vary .difie-rently atthe several receivers with variations -transmiSS i0I.l conditions, ea. receiving circuit, means associated .avith said circuit to automatically select a receiver whose signal outpu exceeds a gpre ssigned i minimum, and means to cause a re-selection of a receiver whenever the slgnal output of the previously selected receiver falls below said minimum.

3. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary difierently at the several receivers with variations in transmission condi tions, a receiving circuit, means associated with said circuit to automatically select a receiver whose signal output exceeds a preassigned minimum, means to cause a re-selection of a receiver whenever the signal out-- put of the previously selected receiver falls below said minimum, and means to cause a re-selection of a receiver after an interruption longer than a preassigned minimum of the signal from a previously selected receiver.

4%; In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary diiferently at the several receivers with variations in transmission conditions, a receiving circuit, and means associated with said circuit to automatically select the receiver having the maximum signal output.

7 5. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary difierently at the several receivers with variations in transmission conditions, a receiving circuit, means associated with said circuit to automatically select the receiver having the maximum signal output, and means to cause a re-selection of a'receiver Whenever the signal output of the previously selected receiver falls below a preassigned minimum.

6. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, means associated with said circuit to automatically select the receiver having the maximum signal output, means to'cause a re-selection of a receiver whenever the signal output of the previously selected receiver falls below a preassigned minimum, and means to cause a re-selection of a receiver after an interruption longer than a preassigned minimum of the signal from a pretions, a receiving circuit, means associated with said circuit to automatically select a receiver whose signal output exceeds a preassigned minimum, and means operating, when more than one of the receivers have substantially the same signal output above said preassigned minimum, to select one of said receivers to the exclusion of the others.

8. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, means associated with said circuit to automatically select the receiver having the maximum signal output, and means operating, when more than one of the receivers have substantially the same maximum signal output, to select one of said receivers to the exclusion of theothers.

9. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary di'lierently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associatedwith each receiver and responsive to signals above a preassigned minimum to establish a transmission connection from said receiver to said receiving circuit, and means operating, when more than one receiver has a signal output above said minimum, to enable the responsive means associated with one receiver to defeatthe establishment of transmission connections from the other receivers.

10. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associated with each receiver and responsive to signals above a preassigned minimum to establish a transmission connection from said receiver to said receiving circuit, means operating, when more than one receiver has a signal output above said minimum, to enable the responsive means associated with one receiver to defeat the establishment of transmission connections from the other receivers, and means to cause a reselection of a receiver whenever the signal output of a receiver previously elfectively connected to said receiving circuit falls below said minimum.

11. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary diflerently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associated with each receiver and responsive to signals above a preassigned minimum to establish a transmission connection from said receiver to said receiving circuit, and means operating when more than one receiver has a signal output above said minimum, to enable the receiver having the largest signal output to defeat the establishment of transmission connections from other receivers.

12. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associated with each receiver and responsive to signals above a preassigned minimum to establish a transmission connection from said receiver to said receiving circuit, means operating, when more than one receiver has a signal output above said minimum, to enable the receiver having the largest signal output to defeat the establishment of transmission connections from other receivers, and means to cause a re-selection of a receiver whenever the signal output of a receiver previously efi'ectively connected to said receiving circuit falls below said minimum.

13. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associated with each receiver and responsive to signals above a predetermined minimum to establish a transmission connection from said receiver to said receiving circuit, and'means operating, when more than one receiver has substantially the same maximum signal output greater than said preassigned minimum, to enable one of the receivers having such maximum output to defeat the establishment of transmission connection from all other receivers.

14. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, signal-responsive means associated with each receiver and responsive to signals above a predetermined minimum to,

establish a transmission connection from said receiver to said receiving circuit, means operating, when more than one receiver has substantially the same maximum signal output greater than said preassigned minimum, to enableone of the receivers having such maximum output to defeat the establishment of transmission connection from all other receivers, and means to cause a re-selection of a receiver whenever the signal output of a receiver previously effectively connected to said receiving circuit falls below said minimum.

15. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary differently at the several receivers with variations in transmission conditions, a receiving circuit, and means associated with said circuit to automatically select a receiver whose signal output conforms to a desired requirement.

In testimony whereof, I have signed in name to this specification this 7th day of December, 1928.

WVILLIAM G. BOHN.

DISCLAIMER 1,747,218.William O. Bohn, Wyoming, N. J. AUTOMATIC SELECTION OF RECEIVING CHANNELS. Patent dated February 18, 1930. Disclaimer filed March 6, 1934, by the assignee, American Telephone and Telegraph Company.

Hereby enters this disclaimer to claims 4 and 15 of said Letters Patent, to Wit:

4. In a signaling system subject to variable transmission conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary diflerently at the several receivers with variations in transmission conditions, a receiving circuit, and means associated With said circuit to automatically select the receiver having the maximum signal output.

15. In a signalingsystem subject to variable transmission Conditions, a plurality of receivers each receiving the same signal and so related to each other that the received signals vary difl'erently at the several receivers with variations in transmission conditions, a receiving circuit, and means associated with said circuit to automatically select a receiver Whose signal output conforms to a desired requirement.

[Ofiicz'al Gazette March 27, 1934.] 

